1. Field of the Invention
This invention relates to a heat-sensitive recording material. More particularly, it relates to a heat-sensitive recording material which comprises a bissulfonate compound, a 2-substituted cyclohexyl sulfonate derivative or a biscycloalkyl aryldisulfonate derivative.
2. Description of the Prior Art
A heat-sensistive recording material is usually produced by applying a colorless or light-colored coloring material and a developer, which induces the coloration of said coloring material upon heating and is separated therefrom, together with other additives, for example, a sensitizer and a binder onto the surface of a substrate such as paper, synthetic paper, plastic film or plastic sheet. When the obtained recording material comes into contact with a heating element such as a thermal head or a heat pen, it shows, for example, a black coloration caused by the reaction between the coloring material and the developer.
Recording materials of the abovementioned type have various advantages such that they require no complicated treatments such as development or fixing, different from other ones; that they are available in recording with the use of a relatively simple device within a short period of time; that they are accompanied by little noise generation and environmental pollution; and that they are inexpensive. Therefore, these recording materials have been widely used not only in copying books and documents but also as a recording material for various instruments including measuring devices, computers, facsimiles, telexes, automatic ticket machines, prepaid cards and labels.
There have been proposed various acidic materials as a developer for thermally inducing the coloration of a coloring material. In particular, phenolic compounds such as bisphenol A, benzyl p-hydroxybenzoate, 4,4'-cyclohexylidenediphenol, 1,1,3-tris(3'-tert-butyl-4'-hydroxy-6'-methylphenylbutane and mixtures thereof have been employed for this purpose. However the use of these phenols in accompanied by some troubles, for example, an insufficient coloring sensitivity, color nonuniformity of reversion, or discoloration upon storage.
Therefore it has been attempted to establish a high coloring sensitivity by using a sensitizer, as the third component, together with a coloring material and a developer. Examples of conventional sensitizers include 4-benzyloxybiphenyl, dimethyl phthalate, stearamide and phenyl benzoate. However each of these sensitizers can not give a satisfactory effect.
It has been further proposed to use a sulfonate derivative as a sensitizer (cf. Japanese Patent Laid-Open No. 73990/1984). This sensitizer improves the coloring sensitivity to a certain extent, though it is yet insufficient. In addition, this method is accompanied by color nonuniformity and reversion and the stability upon storage thereof is unsatisfactory. Furthermore it has been proposed to use a sulfonate derivative as a developer (cf. Japanese Patent Laid-Open No. 109,388/1984). However this developer shows an insufficient printing speed and a low stability upon storage, when used alone. Thus it is required to use an amine compound therewith as a coloring aid. Although the use of the coloring aid is effective in the improvement of the coloring sensitivity, it would lower the stability upon storage. Thus this method is to be further improved.